An optical coupler that uses a lens focusing technique typically comprises several separate lens and optical elements, omen in the form of discrete elements or coatings on one or more of the lenses. Conventional systems having back-to-back graded index lenses with parallel inwardly facing endfaces are known to suffer from an etalon effect due to imperfect refractive index matching of adjacent elements resulting in a distorted output signal. By way of example, in a tap coupler, the tapped signal represents a very small portion of transmitted signal energy passing through a coupling device. The ratio of signal energy to tapped energy is typically in the range of 99:1, thus, any small etalon effect in the optical structure may have a significant effect on the tapped signal in a wavelength dependent manner, causing a "wavelength ripple" on the tapped signal.
Although this problem is known to effect digital systems, its consequences are even more significant in analog optical systems where stringent requirements exist for received signals to have minimal distortion or ripple. It is therefore desirable for both wavelength and non-wavelength dependent passive optical components such as couplers and splitters to provide ripple-free output signals. In attempting to achieve this end, it is an object of this invention to overcome some of the limitations in known prior art devices.
Furthermore, it is an object of the invention to provide a coupler that is more tolerant of imperfect refractive index matching between adjacent interfaces within a coupler.